The objective of this proposal is to determine the complete sequences of the transforming genes of avian sarcoma viruses and to study the mechanism of recombination between viral and cellular sequences in the process of the formation of sarcoma viruses. The genome of Fujinami sarcoma virus (FSV), which contains the sarcomagnetic sequences completely different from that of Rous sarcoma virus (RSV), will be cloned by the recombinant DNA method. The 3' and 5' junctions of FSV-specific sequences and sequences shared with the helper viral RNA will be sequenced. The sarcoma genes (src) of Schmidt-Ruppin strain of RSV (SR RSV) and recently isolated sarcoma virus, called "recovered avian sarcoma virus (rASV)", will also be cloned and sequenced. rASV has been derived from transformation-defective (td) deletion mutants of SR RSV and has been shown to have obtained most of its src from normal chicken cells. Two methods of sequence analysis will be used: one is to employ the method of Maxam and Gilbert using cloned viral DNA, the other method employs essentially the procedure described by Sanger et. al. using chain terminators for DNA synthesis. DNA complementary to the region of viral RNA to be sequenced and RNase T1-resistant oligonucleotides derived from the same region will be used in the second method. The remaining src sequences in td viral RNA which are essential for recombination with cellular sequences to generate the sarcoma virus will be defined and analyzed. This will be done by screening various td mutants for their ability to induce tumors and to generate sarcoma virus, and by analyzing their genomic sequences. The molecular basis of recombination will be assessed by sequence analysis of the junctions of the crossover sites in rASV RNAs using the second sequencing method.